High efficiency hammer mill



Aug. 4, 1964 L. PALYI 3,143,303

HIGH EFFICIENCY HAWIER MILL Filed Dec. 1, 1960 4 Sheets-Sheet 1 INVENTOR LESL/ PAL I by PATENT AGENT HIGH EFFICIENCY HAMMER MILL Filed Dec. 1. 1960 4 Sheets-Sheet 2 o 0 HIV PATENi AGENT Aug. 4, 1964 L. PALYI 3,143,303

HIGH EFFICIENCY HAMMER MILL Filed Dec. 1, 1960 4 Sheets-Sheet 3 O PERCENT OF SCREEN UNOBSTRUCTED l 4 00 BQO I l l l 0 LBS. BRAN MILLED PER HOUR INVENTOR by LE%I? Y! PATE NT AGENT Aug. 4, 1964 PALYl HIGH EFFICIENCY HAMMER mm.

4 Sheets-Sheet 4 Filed Dec. 1, 1960 9. inn

INVENTOR LESL PA Y! by PATENT AGENT rm 1M1 United States Patent 3,143,303 IHGH EFFKCEENCY HAMMER MILL Leslie Palyi, Toronto, ()ntario, Canada, assignor to Universal lviiliing and Machinery Limited, Ottawa, On-

tario, Canada, a corporation of Ontario, Canada Filed Dec. 1, 1960, Ser. No. 72,893 Ciaims priority, application Canada Nov. 4, 1960 16 Claims. (Cl. 241-51) This invention relates to machines for comminuting various materials with high etficiency and more particularly concerns improvements in hammer mills for fine milling of tough or fibrous low density materials.

Heretofore the milling of fibrous or relatively nonbrittle materials by known impact disintegration machines has been relatively unsuccessful and the efficiency, particularly as measured in pounds of finely milled product produced per horsepower-hour of energy used, has been very low. The use of hammer mills for fine milling of forage, hulled cereal grains, and fibrous materials of all kinds has been attended by numerous difiiculties, among which may be listed clogging and plugging of sizing screens, excessive energy consumption, and high temperature rise of milled product. The difficulties of producing very finely divided dry fibrous or tough materials such as Wood flour, bran, forage meal and certain plastics in powder form by impact disintegration, are increased greatly in the presence of moisture and oils due to increased tendency for clogging of screens by milled product and the building up of deposits on surfaces in the comminuting chamber.

I have found that a wide variety of light strong fibrous materials such as coconut fiber, rice hulls, bran, hair and feathers can be successfully milled in the dry state to produce extremely fine particles by providing a high velocity flow of a gaseous fluid under pressure into the comminuting zone of a hammer mill in the direction of circulatory flow within the zone and by maintaining at the same time a relatively high velocity radial flow through the apertures of a cylindrical sizing screen surrounding the zone, particularly when at least part of the inner surface of the screen is formed as a thin coarsely perforated integral layer providing cutting and abrading edges closely adjacent the sizing screen. Moreover the capacity and efficiency of the hammer mill in pulverizing brittle, dense materials is also greatly increased as compared with conventional mills not employing the controlled gaseous fluid flow provisions according to the invention. Wet or oily materials may also be milled provided the liquid content does not exceed about 16% by weight.

Essentially the invention consists in the provision in a hammer mill having a group of toothed striking blades rotatable in an annular comminuting chamber which is circumferentially bounded by a cylindriforrn wall comprising a combined disintegrating and sizing screens and axially bounded by end walls, of at least a pair of apertures in one or both end Walls connected by separate ducts with a high pressure gas supply for injecting a high velocity flow and with a material feeding hopper or the like, and the further provision of suction means for reducing the pressure about the exterior of the screen for augmenting the radial pressure gradient across the screen.

In carrying the invention into effect a hammer mill is constructed with a housing having a pair of spaced apart walls enclosing and supporting between them a cylindrical aper-tured shell wall bounding a comminuting chamber, wherein a system of toothed striking blades supported on a rotor is arranged to be rotated to produce a high tangential velocity of the blades for impacting and tearing fragments of material introduced into the path of the blades through an aperture provided in one or both end walls, part of the inner surface of the shell Wall being 3,1433% Patented Aug. 4., 1964 recessed to assist in disintegration of fragments moving along the curved surface, there being also provided a nozzle structure for directing a blast of high velocity gaseous fluid against the inner surface of the screen at a position spaced angularly downstream of the material infeeding aperture for pressurizing the chamber to increase the velocity of the circulatory flow along the inner surface of the screen.

Preferably the shell wall has a significantly large percentage of its total area apertured and supports on its inner face an auxiliary disintegrating screen formed as a closely adherent apertured metal sheet having apertures whose dimensions exceed by a small multiple the dimension of an aperture in the shell wall and exceed by a large factor the thickness of the metal sheet. The disintegrating screen is arranged to overlie the inner surface of the shell wall along an arc commencing adjacent the infeeding aperture and subtending an angle from about to about 300 degrees or more downstream therefrom, so that disintegration of fragments through impact with the edges of the recessed surface is substantially completed by the time these have traveled along the length of the sheet.

The pressure of the gas supply source supplying gas to the nozzle is made to be substantially greater than the pressure head due to a gas velocity equal to the blade tip velocity, and suction is applied to the space adjacent the exterior of the shell wall. A resultant high velocity circulation developed within the comminuting chamber, in conjunction with a vigorous radially outwardly directed flow of gas through the shell wall, entrains and rapidly removes dust and fine material whose particle dimensions permit it to pass through the sizing apertures of the shell wall, while agglomeration of particles tending to bridge the apertures is prevented by the relatively high pressure gradient maintained across the shell thickness. Coarser fragments are caused repeatedly to strike the edges of apertures in the disintegrating screen and to be rapidly reduced in size thereby and also by being repeatedly cut and torn by the toothed blades. A very rapid comminuting effect is produced which imparts very little heating effect to the particles formed.

In conventional disintegration mills, a pressure differential is maintained between a comminuting Zone adjacent the inner surface of a peripheral screen and its outer surface, by application of suction to reduce the external pressure about the screen to less than atmospheric, while air is supplied to the zone under near-atmospheric pressure. The difference of pressure between inner and outer surfaces of the screen can therefore not be larger than the sum of the pressure difference of the suction device and the velocity head due to circular motion of air in the zone. When the inner surface of the screen becomes slightly obstructed by fragments the differential pressure remains substantially constant, and clogging rapidly ensues, particularly if the orifices are small and the particulate material comprises long fibers of low density.

By the practice of the present invention suction is applied to the exterior of the screen and in addition a jet of gaseous fluid from a relatively high pressure supply is injected continuously into the comminuting zone in sufficient volume to supply substantially the entire flow which passes out through the screen orifices. The amount of flow is arranged to be sufficient to ensure that at the highest rate of material infeeding, the chamber pressure does not rise appreciably above the pressure of the suction device. The fluid jet impinging on the inner surface of the screen has a nozzle discharge velocity which is considerably larger than the blade tip velocity, so that the portion of the screen surface impinged is swept clean of fragments and so that a high speed circulatory flow is maintained adjacent the screen. By the provision of a suitable abrading surface having low relief and the use of an abrading screen pattern elfective to laterally deflect the coarser fragments sliding on it, a highly elfective comminuting action results.

In an operative mill constructed in accordance with the inventive principles the inner surface of the shell wall is kept relatively clear of low velocity particles, the impact of fragment upon fragment is minimized, and the density of milled material occupying a narrow belt or zone immediately adjacent the shell wall is kept low. Frag- -ments and particles are kept in motion at relatively high velocities and with relatively large lateral components, unlike hammer mills of conventional type wherein a significantly larger percentage of the contents of the comminuting zone have low velocity at the screen surface and wherein particles may make a large number of circuits before passing through the screen.

While air at room temperature may be employed as 'the gaseous fluid injected into the comminuting chamber,

other gases or gaseous mixtures may equally well be em ployed, for example an inert gas at a low temperature may be used, with recirculation and suitable conditionmg.

The invention will be more readily understood from the following description of its preferred embodiments, with reference to the accompanying drawing, wherein:

FIGURE 1 shows a hammer mill in partly cut-away perspective illustration, and ancillary high pressure gasimpelling and suction apparatus;

FIGURE 2 shows an elevation side view of the mill of FIGURE 1 in partial section, looking into the comminuting chamber;

FIGURE 3 shows an elevation end view with housing removed and a partial vertical section taken on line 3--3 of FIGURE 1;

FIGURE 4 is a section takenon line 4-4 of the mill of FIGURE 1, showing infeeding and gas injection ports;

FIGURE 5 is a perspective illustration of a portion of a combined disintegrating and sizing screen and a striker blade;

FIGURE 6 shows a section of the screen of FIGURE 5 taken on the line 66 thereof, in enlarged scale; and,

FIGURE 7 is a graph comparing performance of a hammer mill constructed according to the invention with prior art.

Referring to the drawing, material from hopper 10 is fed to the mill, generally designated 11, by any suitable means, as by gravity, the feed rate being regulated by setting wheel 12. An electrovibrator device 13 supported on the hopper assists material to flow at an even rate I through port 14 opening into the comminuting chamber 15 through one wall 16 thereof. The port is located adjacent the path of movement of a series of toothed striking blades 17, radially inward from their tips.

The set of blades is arranged to be independently pivotably supported on rotor 18, the latter being carried by shaft 19 which is journalled in anti-friction bearings 20. A drive motor 21 of any suitable type, for example an electric motor, has its drive pulley 22 coupled by belt 23 with driven pulley 24 fast on the shaft 19. A fiywheel 50 carried on the rotor shaft is provided to maintain constancy of rotational speed, which is such that the blade tip velocity will be preferably of the order of 350 feet per second, or more, thereby to cause entering material to be repeatedly cut and thrown against the surrounding cylindrical screen 25. The latter extends around the comminuting chamber and has a substantial percentage of its area apertured by holes 26 whose size may 7 be from a fraction of a millimeter to several millimeters in diameter.

The comminuting chamber 15 is enclosed at one end by the wall 16 forming part of the machine support structure and at its other end by a door 27, which is hingedly supported on the front wall 28. A flange 29 extending from the back wall 16 has a diameter such that one circular end of the screen may be received therein, while an annular recess 30 formed in the door engages the opposite end of the screen in supporting and end closing relation, permitting the screen to be readily removed and replaced.

Surrounding the screen 25 and spaced outwardly from it is a curved wall 31 connected in gas-tight relation between the opposed surfaces of back wall 16 and front wall 28, to provide a space 32 between the curved wall and the screen. Space 32 is in free communication with aperture 33 leading to exhaust duct 34. The radial separation between screen 25 and wall 31 is arranged to be largest adjacent aperture 33 and least at a point diametrically opposite adjacent the infeeding aperture.

By virtue of the high velocity motion of the set of blades 17, whose outer ends are formed with teeth 35 having a lateral set, a high velocity circular flow of gas is set up in the comminuting chamber, which is attended by a radially outwardly increasing gas pressure gradient. A supply of gas under pressure, which may be air, is injected into the chamber 15 through nozzle aperture 36 in back wall 16 from duct 39, the aperture preferably being spaced on the opposite side of the axis of rotor shaft 19 from the infeeding port 14. The nozzle is so designed and the supply of gas thereto so adjusted that efiluen-t gas has a velocity considerably higher than the tangential velocity of the blade tips and is directed in the direction of their motion, the volume of gas flow per unit time being sufiicient to normally pressurize the chamber at near-atmospheric pressure. Preferably, the opening 14 is located adjacent the inner surface of the screen 25 and the jet is aligned to produce a flow at right angles to a radius drawn through the nozzle from the axis of shaft 19. The gas pressure in the collector space 32 is held at a value lower than the chamber pressure by means of a suction blower 37 connected through cyclone separator 38 with effluent duct 40. The duct 39 is supplied by a high pressure blower 41 suitably driven, as by belt 42 from motor 21, and in a system arranged specifically to recirculate gas, receives its supply from duct 43 connected by a filter-conditioner unit 44 with the delivery side of blower 37.

In one specific system a gas supply source 3.2 pounds per square inch above atmosphere was used while the suction applied at exhaust aperture 33 was of the order of 3 /2 inches of water column below atmosphere. The effect of the joint application of a high velocity jet into .the chamber and suction externally of the chamber is to maintain a high velocity circular flow in an operating 'mill and a high veolcity radial flow through orifices 26 ture may be supplied as atmospheres in which to com minute materials that would be degraded by heat or oxygen, or for pulven'zing substances such as nylon or polyethylene which are relatively non-brittle at room temperature but which embrittle when cooled sufiiciently.

The cylindrical shell screen 25 is doubledover a portion of its length, as shown in FIGURES 4, 5, and 6, by the provision of an inner layer 45 supported closely against the inner surface of the sizing member. The layer 45 preferably comprises a thin, hard, wear resistant metal sheet such as a stainless steel, having a pattern of triangular apertures 46 spaced in a grid comprising parallel ranks of transverse bars 47, oppositely angled oblique bars 48, 49, and longitudinal bars 51, interconnecting in the plane of the sheet. Additional circular apertures 55 are formed in the areas common to the intersection of four bars, to increase the number of abrading edges and to increase the proportion of perforation area to total sheet area.

The nature of the abrading surface presented to fragments of material sliding along the arcuate length of the layer 45 may best be understood from FIGURE 6, wherein it will be assumed that the striking blades 17, of which only the end portions are depicted, are moving away from the observer. The transverse grid bars 47 present edges 52, while oblique edges 53 and 54 are respectively presented by bars 48, 49. The longitudinal bars 51 offer edges 56, 57 to laterally moving particles. In a typical screen member 45 is made of 22 gauge sheet stock from which the apertures are die cut to produce sharp, squarely sheared edges, the sides of the triangles measuring about one centimeter or less. The member 45 is afilxed to the sizing screen after each has been preformed to the desired curvature, by spot welding at intervals along the grid bars with pressure electrodes about two mil limeters in diameter, so that the abrading member lies uniformly in close contact with the sizing screen.

It will be observed that the total length of oblique grid bar edges exceeds the length of transverse bar edges, the effect being to cause fragments to be deflected with substantial lateral component of motion as well as radial inward motion. Since the striking blades are spaced relatively closely a fragment will be struck by the serrations or teeth relatively frequently and will on the average follow an erratic zig-zag course, remaining relatively near to the abrading surface throughout its travel along the member 45. The fragment and its ultimate fractions will be shredded to some degree at each impact with a screen bar edge or with a tooth 35, so that comminution is substantially completed by the time particles leave the abrading member.

The clearance between member 45 and the blade tips is preferably made less than the transverse spacing of the blades in a group, and will generally be of the order of about 9 mm. The radial extent of the comminuting zone has been observed to be relatively shallow, so that the cutting edges formed on the ends of the blades 17 need not extend more than about three-quarters of an inch therealong.

The striking blades preferably have a large number of serrations or teeth 35 formed along their outer ends, these being turned out transversely from both faces of the blade.

Hammer mills constructed according to the invention are characterized by high milling capacity and high efiiciency, as shown by the graph, FIGURE 7, wherein it will be seen that the capacity when fine-milling wheat bran, very greatly exceeds that of a conventional mill with a uniform screen and no forced ventilation. It is evident that the conventional mill is inefiective to comminute such material and consequently plugs rapidly, while the mill according to the invention maintains a relatively unobstructed sizing screen until very heavy feeding rates are reached.

For the highest efficiency of fine milling it is preferable to employ a sizing screen whose ratio of perforation area to total area exceeds 30%.

In one specific embodiment, the screen diameter was 19 inches and had a transverse dimension of 5 inches, and the rotor supported a total of 36 striking blades, the system being driven at 4600 rpm. The sizing screen perforations were 0.37 mm. diameter, and the abrading screen triangular aperture edges had a length between 6 and 8 mm., the abrading layer spanning 180 degrees of arc. Milling rates for oats were consistently of the order of 600 pounds per hour, the product being characterized by extreme fineness of particle free of elongate slivers of hull material.

I claim:

1. In an impact disintegrator apparatus, a rotor journalled for rotation in a frame about an axis and provided with a plurality of circumferentially and axially spaced striking blades pivotally supported in the rotor, a cylindriform apertured sizing screen coaxial with the rotor axis surrounding the rotor and blades, spaced end walls supporting the marginal edges of the screen to enclose a comminuting chamber therebetween, an end wall having an infeeding aperture disposed for feeding material into the chamber at a point intermediate the rotor axis and the outer ends of the blades, and providing a nozzle opening spaced from the infeeding aperture for injecting fluid therethrough tangentially in the direction of blade motion into the comminuting chamber, means to provide a reduced pressure about the exterior of the screen, means supplying said nozzle with a gaseous substance at a pressure greater than the pressure head due to a gas velocity equal to the peripheral blade velocity whereby to produce a jet impinging on a portion of said screen, and an apertured abrading member secured upon the in+ ner surface of said sizing screen and extending in the direction of blade motion from a point adjacent said infeeding aperture over an arc in the range from about to about 300 degrees.

2. An apparatus as claimed in claim 1 wherein said nozzle is disposed to direct said jet away from the end of said abrading member remote from said infeeding aperture.

3. An apparatus as claimed in claim 1 wherein said sizing screen has a net aperture ratio greater than 20%.

4. An apparatus as claimed in claim 1, wherein said nozzle opening is provided in the same end Wall as said infeeding aperture.

5. An apparatus as claimed in claim 2, wherein said axis is horizontal and said infeeding aperture is disposed vertically above said axis.

6. An apparatus as claimed in claim 3, wherein said abrading member comprises a thin metal layer having apertures whose area is a multiple in the range from about 5 to about 50 times the area of the apertures in said sizing screen.

7. An apparatus as claimed in claim 1, wherein one end wall is removable and is provided with an annular shoulder for receiving an end of said screen in coaxial registration with said axis.

8. In a disintegrator, a rotor journalled for rotation in a frame about an axis and provided with a plurality of circumferentially and axially spaced striking blades pivotally supported in the rotor, a cylindrical shell sizing screen circumferentially bounding said rotor and blades and having a uniform radial clearance therefrom, a pair of axially spaced end walls extending radially beyond the margins of said screen and enclosing said screen to provide a comminuting chamber between opposed walls, and connected with a housing spaced outward from said screen to form a receiver space, apertures in said screen communicating with said chamber and said space, an infeeding aperture in an end Wall communicating with said chamber for feeding material, an exhaust aperture opening into said receiver space, gas Withdrawal means connected with said exhaust aperture, and a gas injection aperture in an end wall having nozzle form for delivering a jet of gas to said chamber in the direction of blade motion When connected with a supply of gas under pressure.

9. A distintegrator as claimed in claim 8 wherein said infeeding and injection apertures are provided in one end wall and the other end Wall has a circular aperture through which an end of said cylindrical shell projects, said other end wall being provided with a detachably attachable end closure to permit axial withdrawal of said shell through said circular aperture, said end closure being annularly formed to receive an end of said screen in support relation when said end closure is seated in closing relation on said other end wall.

10. A disintegrator as claimed in claim 8 wherein said injection aperture is spaced circumferentially in the direction of blade motion from said infeeding aperture by an arc distance subtending an angle from about l20.to about 300 degrees about'said axis.

11. A disintegrator as claimed in claim 8 wherein said cylindrical shell screen is laminated over a portion of its length by an abrading member comprising a sheet metal layer closely adjacently attached to said sizing screen and having a grid of triangular openings formed in said layer and communicating. with apertures in said sizing screen, said triangular openings having marginal edges disposed along grid lines which are transverse to, longitudinally aligned with, and obliquely disposed to the length of the member.

12. A combined abrading and sizing screen for an impact idisintegrator comprising a cylindrical sheet metal shell body perforated over its entire circumferential area and supporting on its inner surface an apertured sheet metal layer, said layer having the form of superimposed series of parallel strips whereof the strips of a series are spaced uniformly apart, said series respectively comprising a first series of strips parallel with the axis of said body, a second series of strips perpendicular to the first series, and a third and a fourth series of strips forming diagonals between intersections of adjacent strips of said first and second series, the width of a strip being less than one-fourth the center-to-center distance between adjacent strips.

13. A screen as claimed in claim 12 wherein said apertured layer is further apertured by holes in the areas common to intersections of all of said series of strips.

14. A screen as claimed in claim 12 wherein said sheet metal layer extends along an arc which subtends an angle from about 120 to about 300 degrees at the axis of said cylindric shell body.

15. A disintegrator comprising a rotor provided with a plurality of circumferentially and axially spaced striking blades, a cylindrical shell sizing screen circumferentially bounding said rotor and blades and having a uniform radial clearance therefrom, a pair of axially spaced end walls extending radially beyond the margins of said screen .and enclosing said screen to provide a comminuting chamber between the end walls, and connected with a housing spaced outwardly from said screen to form a receiver space, apertures in said screen communicating said chamber with said space, an infeeding aperture for feeding material through an end wall into said chamber, an exhaust aperture opening into said receiver space, gas withdrawal means connected with said exhaust aperture, a gas inlet in one of the end Walls, said gas inlet being separate from said infeeding aperture, and an external blower arranged to direct a flow of gas through said gas inlet, said gas inlet comprising a gas injection aperture having nozzle form for delivering a jet of gas tosaid chamber in the direction of blade motion, said infeeding aperture and gas inlet being provided in one end wall and the other end Wall having a circular aperture through which an end of said cylindrical shell projects, said other end wall being provided with a detachably attachable end closure to permit axial withdrawal of said shell through said circular aperture, said end closure being annularly formed to receive an end of said screen in supporting relation when said end closure is seated in closing relation on said other end wall.

16. A disintegrator as claimed in claim 15 wherein said injection aperture is spaced circumferentially in the direction of blade motion from said infeeding aperture by an arc distance subtending an angle from about to about 300 degrees at said axis.

References Cited in the file of this patent Jacobson June 2, 1953 

1. IN AN IMPACT DISINTEGRATOR APPARATUS, A ROTOR JOURNALLED FOR ROTATION IN A FRAME ABOUT AN AXIS AND PROVIDED WITH A PLURALITY OF CIRCUMFERENTIALLY AND AXIALLY SPACED STRIKING BLADES PIVOTALLY SUPPORTED IN THE ROTOR, A CYLINDRIFORM APERTURED SIZING SCREEN COAXIAL WITH THE ROTOR AXIS SURROUNDING THE ROTOR AND BLADES, SPACED END WALLS SUPPORTING THE MARGINAL EDGES OF THE SCREEN TO ENCLOSE A COMMINUTING CHAMBER THEREBETWEEN, AN END WALL HAVING AN INFEEDING APERTURE DISPOSED FOR FEEDING MATERIAL INTO THE CHAMBER AT A POINT INTERMEDIATE THE ROTOR AXIS AND THE OUTER ENDS OF THE BLADES, AND PROVIDING A NOZZLE OPENING SPACED FROM THE INFEEDING APERTURE FOR INJECTING FLUID THERETHROUGH TANGENTIALLY IN THE DIRECTION OF BLADE MOTION INTO THE COMMINUTING CHAMBER, MEANS TO PROVIDE A REDUCED PRESSURE ABOUT THE EXTERIOR OF THE SCREEN, MEANS SUPPLYING SAID NOZZLE WITH A GASEOUS SUBSTANCE AT A PRESSURE GREATER THAN THE PRESSURE HEAD DUE TO A GAS VELOCITY EQUAL TO THE PERIPHERAL BLADE VELOCITY WHEREBY TO PRODUCE A JET IMPINGING ON A PORTION OF SAID SCREEN, AND AN APERTURED ABRADING MEMBER SECURED UPON THE INNER SURFACE OF SAID SIZING SCREEN AND EXTENDING IN THE DIRECTION OF BLADE MOTION FROM A POINT ADJACENT SAID INFEEDING APERTURE OVER AN ARC IN THE RANGE FROM ABOUT 120 TO ABOUT 300 DEGREES. 